Silver nanoparticle materials have become increasingly important in many technologies due to silver's unique chemical stability, excellent electrical conductivity, catalytic activity, and antimicrobial effect. Silver nanoparticle materials have found uses in microelectronics, optical, electronic and magnetic devices, sensors, especially in biosensors, and catalysts. For example in the area of printable electronics, silver nanoparticle dispersions have been widely regarded as the best candidates to form conductive traces by solution deposition processes. The solution processes permit a roll-to-roll process on flexible substrates at mild temperatures, which significantly reduces cost. The excellent conductivity of silver makes it possible to form very fine patterns (e.g., mesh or grid patterns) of conductive micro-wires that are essentially transparent to the unaided eye.
The proposed solution processes of the prior art include inkjet printing, micro-contact printing, flexographic printing, gravure printing, and direct ink-wiring through fine nozzles onto a substrate. With flexographic and gravure printing processes, the typical wet coverage (thickness) is on the order of a couple of microns, especially for narrow micro-wires. In order to accomplish greater than 0.3 μm dry coverage, the metal particle concentrations in the inks has to be greater than about 15% by volume, which is equivalent to a greater than 65% by weight.
For certain applications such as RFID tags, ink jet printing can be used to generate features having of low aspect ratios, e.g. less than 0.5 micron thick and greater than 50 microns wide. U.S. Pat. No. 8,227,022 has disclosed the generation of conductive patterns using aqueous based silver nanoparticle inks with multi-pass ink jet printing (5 passes or more) and sintering the printed patterns at temperatures of equal to greater than 150° C. The electrical resistivity generated at such conditions is greater than 0.2 ohms/square. The requirement of multi-pass and the resultant poor conductivity are perhaps due to the low weight percentage of silver nanoparticles in the inks and the particular stabilizers used which could lead to poor curing of silver nanoparticles during sintering.
U.S. Pat. No. 7,922,939 discloses a silver nanoparticle ink composition having a silver concentration of greater than 50% by weight and containing a first anionic polymer stabilizer having a molecular weight of at most 10,000, and a second anionic polymer stabilizer having a molecular weight of at least 25,000. The inks disclosed can be considered as a high viscous gel and have an elastic modulus value greater than the loss modulus value. Such inks are useful for deposition process such as direct ink writing. However the electrical conductivity generated by such ink compositions is limited after annealing at high temperatures.
U.S. Pat. No. 7,931,941 discloses a method of making silver nanoparticle dispersion using a carboxylic acid stabilizer having from 3 to 7 carbons. Such dispersions can be sintered into conductive films at lower sintering temperatures. However the dispersions are not water reducible and cannot be formulated into ink-jet inks.
Jung et al, Morphology and conducting property of Ag/poly(pyrrole) composite nanoparticles: Effect of polymeric stabilizers, Synthetic Metals 161 (2011) pgs. 1991-1995 discloses silver/polypyrrole composite nanoparticles prepared using poly(4-styrenesulfonic acid-co-maleic acid) sodium salt or poly(N-vinylpyrrolidone) as stabilizer. These formulations have low % Ag content, high ratios of stabilizer to silver and resistivities tens or hundreds of times higher than bulk silver.
WO2010/109465 discloses incorporating halide ions as a sintering agent into silver dispersions or receivers to improve conductivity.
WO2009/081386 discloses a method for producing very dilute solutions of silver nanoprisms that are not suitable for electronic devices.
There are various forms of non-aqueous based silver nanoparticle dispersions which have been described in the prior art. Some of them are commercially available. For environmental and safety reasons, it is highly desirable to have aqueous-based silver nanoparticle dispersions. For performance reasons, it is highly desirable that these aqueous silver nanoparticle dispersions are colloidally stable, can be prepared at high concentrations with low viscosities, are water reducible with excellent re-dissolution behaviors, and have excellent electrical conductivity after sintering.